docs: def _concatenate_spike_clusters(*pairs):
      """Concatenate a list of pairs (spike_ids, spike_clusters)."""
      pairs = [(_as_array(x), _as_array(y)) for (x, y) in pairs]
      concat = np.vstack(np.hstack((x[:, None], y[:, None]))
                         for x, y in pairs)
      reorder = np.argsort(concat[:, 0])
      concat = concat[reorder, :]
      return concat[:, 0].astype(np.int64), concat[:, 1].astype(np.int64)